If the moon is crashing down to Earth, how would you save Earth?

Assume anything that you want to assume to make this as fun/intriguing an exercise as you want. Time, resources, your job, connections, whatever, as long as they're not supernatural or superhuman. In other words, some semblance of science should probably remain.

An atom bomb would do nothing helpful at all. Launching the largest hydrogen bomb ever tested (Tsar Bomb 50MT) would not help at all much more. This would be like firing a bottle rocket at a football stadium that was hurled at you at 100 MPH. At best, you would throw up a little dust.